Historical Constraints and the Evolution of Development 1077
expresses an opinion as to how the similarity did not arise, i.e., that it did not
arise by homology, but it does not express an opinion as to how the similarity
did arise. I do not... see these as alternatives at the same categorical level. The
set is not positive, "a" and "b" as mutually exclusive categories, but is a
dichotomy of "a" and "not-a." Under "not-a" it is still possible to have a
sequence of alternatives, such as "b," "c," etc., that are positive categories on
the same level as "a."The standard literature does include a venerable term—analogy—that might
establish a contrast with homology in the causal sense that wins our almost visceral
assent as more satisfactory, with homology as positive A (similarity due to common
descent, with no need to invoke direct selective molding), and analogy as oppositely
positive B (similarity due to common pressures of natural selection upon
backgrounds of no common descent).
We now encounter the logical dilemma that underlies nearly all our extensive
and lamentable confusion on this issue. Homoplasy and analogy might strike us, at
first, as fully synonymous, for both invoke natural selection as the source of separate
evolution for similar structures in two lineages. This synonymy certainly applies for
convergence. But homoplasy comes in two flavors: parallelism and convergence—
with parallelism as the historical root (in Lankester's original definition of
homoplasy), but only convergence carrying the full flavor of synonymy. That is,
convergence stands opposite to homology by both criteria—the negative not-A of
origin not by common descent, and the positive B of origin by natural selection
working in a similar way upon two unrelated substrates.
Unfortunately, a common error of human thinking leads us to define broad and
variable categories by their clearest extreme cases. Thus, many scientists have
assumed that all homoplasy, whether by parallelism or by convergence, must
originate entirely for functional reasons, and not at all by constraint (the B category
of exclusively dichotomous logic); whereas, the "not-A" of independent origin
identifies the only property truly required for inclusion within the broad definition of
homoplasy. Simpson continues (in Simpson and Haas, 1946, ibid.):
Moreover, the implication is usually present to some degree and it has
sometimes been explicitly stated that the structural similarity here in question
is not due to homology but is correlated with community of function as
opposed to community of ancestry. It is in this sense that analogy is a true
alternative (but not the only alternative) to homology as a positive category on
the same level, a "b" category rather than a "not-a" category or something on a
different level altogether. That is, analogy, when used in this way, expresses a
positive opinion, or theory, that a structural resemblance is correlated with
function, just as homology expresses the view that it is correlated with
common ancestry. Unlike homoplasy, analogy offers an alternative theory as
to the basis of the resemblance in question.